Simple support for switching to a different postgres version

[cprice404]

I believe that at the time being, if you'd like to switch to a different version of postgres (e.g. from 8.4 to 9.1), you probably need to override a bunch of different path variables in your manifest. It would be nice to have a simple catch-all version-number setting, which would automatically handle the logic of switching all of the path variables.

[cprice404]

This should be mostly working as of pull req #43.